1. Field of the Invention
The present invention relates to a polarizing plate protection film used for protecting a polarizer of a polarizing plate, a polarizing plate using the polarizing plate protection film, and a liquid crystal display device using the polarizing plate.
2. Description of the Related Art
In recent years, liquid crystal displays have been rapidly spread in place of conventional CRT displays, because of their characteristics such as electric power saving, lightweight and slimness. As shown in FIG. 11, for example, a general liquid crystal display device 100 comprises a backlight-side polarizing plate 102A, a display-side polarizing plate 102B, and a liquid crystal cell 101. In this structure, the polarizing plates 102A and 102B are each so configured that only linearly polarized light having an oscillation surface in a specific oscillation direction can be selectively transmitted, and placed opposite to each other in the crossed-Nicol arrangement so that the respective oscillation directions can be orthogonal to each other. The liquid crystal cell 101 is placed between the polarizing plates 102A and 102B.
Known examples of such liquid crystal display utilize various driving modes depending on the alignment mode of the liquid crystal material in the liquid crystal cell. The majority of currently popular liquid crystal display devices are classified into TN, STN, VA, IPS, and OCB modes. In particular, VA-mode and IPS-mode liquid crystal display devices have become widely spread today.
The polarizing plate used for the liquid crystal display device generally has a technical structure where a polarizer is sandwiched between two polarizing plate protection films. A film made of cellulosic ester exemplified by triacetylcellulose has been conventionally used as the polarizing plate protection film. This is due to advantages of cellulosic ester such as follows: it has a low in-plane retardation (Re) value so that effect to the display quality of the liquid crystal display device is small, and it has appropriate water permeability so that it can dry through the polarizing plate protection film the moisture remained to the polarizer at the time of producing a polarizing plate. Also, the relatively moderate price of cellulosic ester film is also a contributing factor.
As explained, since the cellulosic ester film has properties suitable for the polarizing plate protection film, a polarizing plate protection film made of cellulosic ester film has been generally used as the polarizing plate for a liquid crystal display device.
However, as a member to catalyze the developing technical field of liquid crystal display devices, using of such cellulosic ester film encompass various problems. As particularly big problems, the followings have been noted.
First, since cellulosic ester generally has no melting point, a cellulosic ester film is generally produced by a so-called solution film-forming method in which a solution with cellulosic ester dissolved in an organic solvent is casted on a supporting body, the solvent is dried and then peeled from the resultant. However, large-scale equipments and specialized techniques are required to carry out such solution film-forming method including the solvent drying process and the like. Thus, the producer of the cellulosic ester film is limited to those who possess the specialized techniques and the production of the cellulosic ester film cannot meet the demands of the developing liquid crystal display device technical field. Therefore, continuing to relying on the cellulosic ester film as the polarizing plate protection film has a danger of hindering the development in the liquid crystal display device technical filed, which should be regarded as one of the major industries of Japan without exaggeration.
Further, in generally forming a cellulosic ester film by the solution film-forming method, dichloromethane is used for a main solvent as an organic solvent for the cellulosic ester solution. However, dichloromethane is suspected its risk to the human body. As such, it is not desirable from environmental aspect to continue relying on the cellulosic ester film for years to come because it will increase the consumption and discharge of dichloromethane as the liquid crystal display device technical filed develops.
In view of this, a film substituting the cellulosic ester film is desired today to use as a polarizing plate protection film. Especially from the view point of the above-explained problem casted by the cellulosic ester film, using of a general-purpose film which is easily available in the market or capable of being produced by a simple method is desired as a polarizing plate protection film. Based on such consideration, attempts have been made to utilize a film substituting the cellulosic ester, such as a film made of polyethylene terephthalate (for example, Japanese Patent Application Publication (JP-A) No. 2004-205773).
When using a film having high general-purpose properties, such as one made of polyethylene terephthalate (PET) as a cellulosic ester film-substituting film, there is a problem that a display quality of a liquid crystal display device is impaired. This is caused by a problem that a film made of polyethylene terephthalate or the like having high general-purpose properties generates irregularities having different colors (hereinafter, they are sometimes referred “rainbow-like irregularities”, they are especially observed when viewed from an oblique direction) to a liquid crystal display device compare to a case of using a conventional cellulosic ester film.
Thus, in case of using a general-purpose film of polyethylene terephthalate or the like instead of a cellulosic ester film, it is necessary to use a film at least having a low in-plane retardation (Re) value. However, a general-purpose film of polyethylene terephthalate or the like generally has very large in-plane retardation (Re) due to a material or film-forming method and a specialized technology improvement is necessary to reduce the in-plane retardation (Re) value. Thus, the effect of using a general-purpose film instead of a cellulosic ester film becomes eliminated and the usage of such a general-purpose film becomes to have no contribution to the resolution of the facing problem.